Planar inverted F antennas have been used as antennas for a wireless communication unit provided on a circuit board of communication modules, e.g., mobile telephones, wireless LAN (Local Area Network) appliances. These antennas are build-in antennas provided on a circuit board with a relatively low profile, employing the circuit board for grounding. Planar inverted F antennas are applied to various types of communication modules, since planar inverted F antennas include a plurality of planar elements, which can be manufactured from low-cost metal plates, and are easily attached to a circuit board.
For example, the technique related to Planar inverted F antennas is disclosed in following Patent Reference 1.    Patent Reference 1: Japanese Laid-open Patent Publication No. 2008-263468
As an example of a planar inverted F antenna, a planar inverted F antenna 200 is depicted in FIG. 1.
The planar inverted F antenna 200 includes a planar grounding element 100 that is to be placed on a GND surface of a circuit board, a planar radiation element 120 (having a length L1 and a height H) extending substantially parallel to the grounding element 100, and planar short-circuit elements 140 and 160 that short-circuit the grounding element 100 and the radiation element 120. A power supply section F that applies wireless signals from the circuit board is provided at the short-circuit element 160. The planar inverted F antenna 200 has literally an inverted F geometry.
FIG. 2 indicates the planar inverted F antenna 200, provided on a GND surface of the circuit board. As depicted in FIG. 2, the grounding element 100 of the planar inverted F antenna 200 is attached to the GND surface having a size of K1×K2 (on the X-Z plane). As depicted in FIG. 2, the planar inverted F antenna 200 may be provided at the end of circuit board so as not to interfere with other components provided on the circuit board.
FIGS. 3A and 3B indicate electromagnetic field simulator results of the planar inverted F antenna 200, wherein FIG. 3A indicates the voltage standing wave ratio (VSWR) characteristic, and FIG. 3B indicates the directional property on the X-Y plane, of the planar inverted F antenna 200 when the antenna 200 is provided on the circuit board as in FIG. 2. It is noted that FIGS. 3A and 3B indicate the results when L1 is 70 mm, H is 9 mm, the spacing between the short-circuit elements is 4 to 5 mm, the width is each of the short-circuit elements is 2 mm, the plate thickness of each antenna element is 0.4 mm, and K1=K2=70 mm in FIG. 2. It is clear from FIGS. 3A and 3B that this planar inverted F antenna 200 exhibits a favorable omnidirectional characteristic, while the bandwidth remains about 25 MHz at VSWR of 2.
The height of the radiation element 120 (height H in FIG. 1) of the planar inverted F antenna 200 with respect to the grounding element 100 cannot be increased any further, due to the size limitation of a casing of a communication module to which the antenna is to be accommodated, which hinders further extension of the bandwidth of the antenna.